Organoborane based systems are known to initiate free radical polymerization and promote adhesion to low surface energy substrates due to their ability to generate radicals to polymerize compounds capable of free radical polymerization such as compounds containing unsaturated moieties. The oxidation of organoborane based systems with molecular oxygen forms energetic peroxides which are exothermic and can be pyrophoric if not carefully controlled. Due to the high reactivity of organoboranes with oxygen, systems have been developed which block the organoborane center to stabilize the organoborane and which unblock the organoborane center to initiate free radical formation. The role of the blocking group is to render the organoborane center less susceptible to oxygen insertion and radical initiation.
EP 1,201,722 discloses the use of L-selectride and phenyl borate as the borane precursors with hydride and phenyl anions as the blocking groups. A series of patents issued to Skoultchi, U.S. Pat. Nos. 5,106,928; 5,143,884; 5,286,821; 5,310,835 and 5,376,746 (all incorporated herein by reference) and to Zharov, et al., U.S. Pat. Nos. 5,539,070; 5,690,780; and 5,691,065 (all incorporated herein by reference) disclose polymerizable acrylic compositions which are particularly useful as adhesives wherein organoboron amine complexes are used to initiate cure. Pocius in a series of patents, U.S. Pat. Nos. 5,616,796; 5,621,143; 5,681,910; 5,686,544; 5,718,977; and 5,795,657 (all incorporated herein by reference) disclose amine organoboron complexes using a variety of amines to complex the organoboron, such as polyoxyalkylene polyamines and polyamines which are the reaction product of diprimary amines and compound having at least two groups which react with a primary amine. A series of patents by Sonnenschein et al. U.S. Pat. Nos. 6,806,330; 6,730,759; 6,706,831; 6,713,578; 6,713,579 and 6,710,145, disclose amine organoboron complexes wherein the organoboron is a trialkyl borane and the amine is selected from the group of amines having an amidine structural component; aliphatic heterocycles having at least one nitrogen in the heterocyclic ring; an alicyclic compound having bound to the ring a substituent having an amine moiety; primary amines which in addition have one or more hydrogen bond accepting groups wherein there are at least two carbon atoms between the primary amine and the hydrogen bond accepting group; and conjugated imines. These patents disclose polymerizable compositions containing the amine organoboron complexes, one or more of monomers, oligomers or polymers having olefinic unsaturation which are capable of polymerization by free radical polymerization and that the polymerizable compositions can be used as adhesive, sealant, coating or ink compositions. Kendall et al., U.S. Pat. No. 6,630,555 (incorporated herein by reference) discloses useful boron containing compounds for initiating polymerization are internally blocked organoborates, incorporated herein by reference. The term “internally blocked” in reference to the organoborates is described as a four coordinate boron atom being part of an internal ring structure bridged across two of the four boron coordinates or valences. Kneafsey et al., U.S. Publication Number 2003/0226472 and Kneafsey et al., U.S. Publication Number 2004/0068067 disclose another class of organoborates useful in initiating polymerization is tetrahydrocarbyl borates (also known as quaternary boron salts), both incorporated herein by reference.
Low surface energy olefins such as polyethylene, polypropylene and polytetrafluroethylene have a variety of attractive properties in a variety of uses, such as toys, automobile parts, furniture applications and the like. Because of the low surface energy of these plastic materials, it is very difficult to find adhesive compositions that bond to these materials. The commercially available adhesives that are used for these plastics require time consuming or extensive pretreatment of the surface before the adhesive will bond to the surface. Such pretreatments include corona treatment, flame treatment, the application of primers, and the like. The requirement for extensive pretreatment of the surface results in significant limitations to the designers of automobile components, toys, furniture and the like.
There is a continued need for free radical polymerization initiator systems that are stable under storage conditions, which initiate rapid cure once polymerization is initiated and which are capable of bonding to low surface energy substrates.